1. Field of the Invention
The field of art to which the claimed invention pertains is processes for treating sour petroleum distillates. More specifically, the claimed invention relates to the treatment of sour petroleum distillates with an oxidizing agent in the presence of an oxidation catalyst to effect the oxidation of mercaptans in the distillate to disulfides.
2. Description of the Prior Art
Processes for the treatment of sour petroleum distillates wherein the distillate is treated in contact with an oxidation catalyst in the presence of an oxidizing agent at alkaline reaction conditions have become well known and widely practiced in the petroleum refining industry. Said processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour petroleum distillate with the formation of innocuous disulfides--a process commonly referred to as sweetening. Depending on the source of the petroleum from which the sour distillate was derived, the boiling range of the distillate itself, and possibly the method of processing the petroleum to produce the distillate, the distillates vary widely with respect to the concentration, molecular weight and complexity of the mercaptans contained therein, and the sweetening process will vary accordingly. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour petroleum distillate. Other sour petroleum distillates include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, lube oil, and the like.
One such process relates to olefin-containing petroleum distillates. When said distillates are required to be maintained in storage for any length of time, they advantageously contain an oxidation inhibitor to obviate gum formation. The inhibitor is typically an oil-soluble phenylenediamine. When the olefin-containing distillates further contain a relatively small concentration of the more readily oxidizable mercaptans, the phenylenediamine acts as a homogeneous oxygen transfer agent and, in the presence of an alkaline reagent, promotes the oxidation of mercaptans and the formation of disulfides. It is to be noted that at least one-third of the mercaptans are consumed by interaction with the olefin content of the sour distillate. The process is commonly referred to as inhibitor sweetening. The homogeneous phenylenediamine is not recoverable but is expended in the sweetening process, and as the amount of the phenylenediamine required to effect an economical rate of oxidation becomes excessive, the process becomes ineffective as a sweetening process and resort must be had to other means. It is known that inhibitor sweetening, which is essentially a batch type of process more suited to the treatment of sour distillates in storage, functions only with respect to olefin-containing distillates--the olefin being essential to the inhibitor sweetening process. Over a period of time, usually measured in hours or days, the stored distillate may become doctor sweet depending on the complexity and the concentration of the mercaptans contained therein. While certain quaternary ammonium compounds have been used in conjunction with the homogeneous phenylenediamine catalyst to accelerate the sweetening process as shown in U.S. Pat. No. 3,164,544, the process remains subject to the general limitations of inhibitor sweetening, i.e., ineffective with respect to sour petroleum distillates containing mercaptans other than primary and secondary mercaptans, increasingly ineffective with respect to petroleum distillates containing in excess of about 150 ppm. mercaptan sulfur, and restricted to batch processing procedures rather than the commercially more desirable continuous processing methods.
A commonly used continuous treatment process for sour petroleum distillates entails treating the distillate in contact with a heterogeneous metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour distillate and the catalyst-containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. The continuous type of operation requires a substantially lesser contact time than required of inhibitor sweetening. The metal phthalocyanine catalyst, which is recovered and recycled for continuous use, is not limited to use in conjunction with an olefin-containing petroleum distillate, but is equally effective with regard to olefin-free distillates to provide a doctor sweet product.
Certain of the higher boiling sour petroleum distillates, generally boiling in excess of about 275.degree. F., contain highly hindered branched chain and aromatic thiols, and/or higher molecular weight tertiary and polyfunctional mercaptans, which are at most only partially soluble in the catalyst-containing caustic solution of the liquid-liquid treating system. Sour petroleum distillates containing these more difficulty oxidizable mercaptans are more effectively treated in contact with a metal phthalocyanine catalyst disposed on a high surface area adsorptive support--usually an activated charcoal. The distillate is treated in contact with the supported metal phthalocyanine catalyst at oxidation conditions in the presence of an alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the distillate to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in a caustic-wetted state.
Heretofore, the practice of catalytically treating mercaptan-containing sour petroleum distillates has involved the introduction of alkaline agents, usually sodium hydroxide, into the sour petroleum distillate prior to or during the treating operation. (U.S. Pat. No. 3,108,081; U.S. Pat. No. 4,156,641). The prior art also suggests the addition to the petroleum distillate along with certain alkaline agents of certain non-alkaline additives. (U.S. Pat. No. 4,124,493; U.S. Pat. No. 4,033,860). In addition, the prior art suggests the use in an alkaline environment of certain catalytic composites produced from metal phthalocyanine solutions containing certain non-alkaline additives. (U.S. Pat. No. 4,087,378; U.S. Pat. No. 4,124,531).
What has been needed, but which the prior art does not suggest, is a process for treating sour petroleum distillates using a catalystic composite of sufficient activity that addition to the petroleum distillate of an alkaline agent is not required. The method of this invention is such a process. The prior art does not disclose or suggest the treating of a mercaptan-containing sour petroleum distillate by contacting the distillate at oxidation conditions with an oxidizing agent and a catalytic composite comprising a metal chelate and a quaternary ammonium hydroxide disposed on an adsorptive support. The treating process of this invention can be accomplished without the necessity of addition of an alkaline agent with consequent savings in materials handling and storage expenses, and avoidance of use of hazardous alkaline chemicals in the treating process.